On the code division multiple access CDMA air interface radio resources are shared between different users by means of a combination of scrambling codes and channelization codes. In the downlink different code trees are generated by applying different scrambling codes to at least one channelization code tree.
As shown in FIG. 1, to each scrambling code there is related a code tree with a set of channelization codes also referred to as codes in the following that are used to separate different channels in a cell of the CDMA cellular mobile communication network. Typically, these channelization codes are orthogonal variable spreading factor OVSF codes that preserve the orthogonality between different downlink channels at different transmission rates. Particularities with respect to scrambling codes and channelization codes are described in, e.g., 3G TS 25.213 V3.1.1 “Spreading and modulation (FDD)” incorporated herein by reference.
As shown in FIG. 1, for each channelization code there is also defined a related spreading factor. E.g., a speech service of 8 kbps may use a spreading factor of 128 (not shown in FIG. 1) and a 384 kbps packet switched data service may require a much lower spreading factor of 8. Therefore, the higher the required bandwidth for a specific downlink channel the lower the number of available downlink channels. Considering also that common control channels consume part of the available radio resources, the maximum number of users in the mobile cellular communication network further drops to lower values.
Therefore, it is expected that the number of available channelization codes will often be a limiting factor in providing downlink channels in CDMA cellular mobile communication networks, i.e. that code limitation will restrict the access of users to the network.
In other words, these CDMA cellular mobile communication networks will perceive code limitation in particular when there are many packet data users in the system requiring high bandwidth and when additional capacity enhancing techniques are applied, e.g., space-time coding, interference cancellation, and/or adaptive antennas.
However, sometimes it is not the downlink channelization codes that are the limiting factors, but rather the power available for data transmission. Nevertheless, the available power is “soft limited” meaning that a temporary increase of power for data transmission over a certain threshold may be tolerated to a certain extent. On the other hand, channelization codes are “hard limited” meaning that, in case there are not enough codes, data transmission is impossible.
When using one scrambling code and evaluating code limitation against power limitation, it would appear that downlink power limitation only occurs if most channelization codes are used at the same time. However, this assumption is true only when many users are active at the same time. Further, in real life—in particular due to the burstiness of packet data connections—channelization codes will be allocated although they are not used. Therefore, this approach leads to a situation where the system is code limited while the maximum power limit is not reached. Nevertheless, new users may not access downlink channels in the CDMA cellular mobile communication network.
A first approach known from the state of the art to overcome this problem is code management. By using radio resource control signaling the downlink channelization codes are constantly assigned and re-assigned between users in the cells according to their activity. However, this requires an immense radio resource control signaling further increasing the load in a cell of the mobile communication cellular network. Further, resource allocation is slow and requires a time larger than the round trip time on the radio link, i.e. a code allocation through radio resource control takes approximately 500 ms. However, this is too slow especially for high data rate end users.
A second approach known from the state of the art is the use of a downlink shared channel DSCH. Here, several users share the same channelization code assigned to the downlink shared channel and resources are assigned in a time multiplexed manner. While the radio resource control signaling load compared to the previous code management approach is reduced, code management is still needed to adapt the capacity of the downlink shared channel to the load imposed by several users. Also, a soft handover is not possible on this downlink shared channel and an associated downlink channel is needed for each single user equipment terminal for the control signaling.
Yet another approach according to the prior art is the use of multiple scrambling codes, i.e. the use of multiple code trees. This approach enables the assignment of several scrambling codes to downlink channels to solve the code limitation outlined above and therefore allows for a reduced radio resource control signaling load compared to the code management approach. A major drawback of the use of multiple code trees is, however, that it causes a large increase in interference due to loss of orthogonality of used channelization codes. In other words, channelization codes of different code trees are not orthogonal with respect to each other and therefore the interference level in the cell of the mobile cellular communication network increases.